1. Field of the Invention
The field of the invention is fluidized catalytic cracking (FCC) in general and catalyst stripping in particular.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants in catalytic cracking: moving bed and the far more popular and efficient fluid bed process.
In fluidized catalytic cracking (FCC), catalyst, having a particle size smaller than, and color resembling, table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating it. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. A catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone much development since the 40s. The trend of development of the FCC process has been to all riser cracking and zeolite catalysts.
Riser cracking gives higher yields of valuable products than dense bed cracking. Most FCC units now use all riser cracking, with hydrocarbon residence times in the riser of less than 10 seconds, and even less than 5 seconds.
Zeolite based catalysts of high activity and selectivity are now used in most FCC units. These catalysts allowed refiners to increase throughput and conversion, as compared to operation with amorphous catalyst. The zeolite catalyst effectively debottlenecked the reactor section, especially when a riser reactor was used.
Another development occurred which debottlenecked the FCC regenerator--CO combustion promoters. To regenerate FCC catalysts to low residual carbon levels refiners used to add limited amounts of air. Coke was burned to CO and CO2, but air addition was limited to prevent afterburning and damaging temperature excursions in the regenerator. U.S. Pat. Nos. 4,072,600 and 4,093,535, which are incorporated by reference, taught adding Pt, Pd, Ir, Rh, Os, Ru and Re in concentrations of 0.01 to 50 ppm, to allow CO combustion to occur within the dense bed of catalyst in the regenerator. CO emissions were eliminated, and regenerators were now limited more by air blower capacity than anything else.
To summarize, zeolite catalysts increased the capacity of the cracking reactor. CO combustion promoters increased the capacity of the regenerator to burn coke. FCC units now had more capacity, which could be used to process worse feeds or achieve higher conversions. Constraints on the process, especially for units already in operation, could now shift to some other place in the unit, such as the wet gas compressor, main column, etc.
One way refiners took advantage of their new reactor and regenerator capacity was to process feeds that were heavier, and had more metals and sulfur. These heavier, dirtier feeds pushed the regenerator, and exacerbated existing problems in the regenerator--steam and temperature. These problems show up in the regenerator and are reviewed in more detail below.